The mammalian heart, the first functional organ in the developing vertebrate embryo, includes three distinct structures—the epicardium, myocardium, and endocardium. The epicardium, the outermost layer of the heart, contributes both multi-lineage descendants and important trophic signals to the myocardium and coronary vessels. The epicardium develops from the proepicardium, a mass of coelomic progenitors located at the venous pole of the embryonic heart. Proepicardium cells attach to and spread over the myocardium to form the primitive epicardial epithelium. During cardiogenesis, epicardial cells undergo epithelial-to-mesenchymal transition (EMT) to give rise to a population of epicardium-derived cells, which in turn invade the heart and progressively differentiate into various cell types, including cells of coronary blood vessels and cardiac interstitial cells.
The epicardium contributes both multi-lineage descendants and paracrine factors during cardiac repair, underscoring their potentials for regenerative medicine (Lepilina et al., Cell. 127:607-619 (2006); Kikuchi et al., Dev. Cell. 20:397-404 (2011); Zhou et al., J. Clin. Invest. 121:1894-1904 (2011); Zhou & Pu, J. Cell. Mol. Med. 15:2781-2783 (2012)). Understanding the molecular mechanisms that control the specification of epicardial lineages from naïve progenitor cells is fundamental to elucidating the regulatory mechanisms underlying both human heart development and cardiovascular diseases. Because of their developmental importance and therapeutic potential, epicardial cells and epicardium-derived cells present a tractable resident progenitor source to restore a functional vasculature, to maintain cardiomyocyte survival, and to repair damaged heart tissue. Accordingly, there is a need in the art for efficient and cost-effective protocols for generating functional epicardial cells under chemically defined culture conditions and in the absence of certain growth factors previously thought to be an essential part of directed epicardial differentiation.